It is one type of end-fire array and based on the dipole
element. It is comprised of a set of dipole and dipole elements vary in size
from smallest at the front to largest at the rear. A balanced feedline is
connected to the narrow end, and power is fed to the other dipoles via a network
of crossed connections. This arrangement creates an active region of adjacent
dipoles that accepts power at any given frequency.
Because it's characteristic of closely matches the 377
ohms impedance of free space, therefore it contribute to its efficiency. This
type of antenna has a linear polarization and its typical gain is between
6 to 8 dB. LPDA also has a frequency limit of 30 MHz until 1.8 GHz. The antenna
is constructed of aluminium to minimize weight and reduce windloading. This
antenna also takes advantages of its relatively high front-to-back gain ratio.
Log-periodic dipole array is most commonly used for television reception.
LPDA also used in radio reception because LPDA tend to have more uniform gain
over the FM band and wider beamwidths.
LPDA also is a beam antenna optimized for wide frequency
bandwidth. It is decreasing size (with the longest in back and the smallest
in front), connected to a boom, to hold them a specific distance apart. As
the number and lengths of elements increases, the operating bandwidth increases.
Specifically, the theoretical values of the log-periodic are shown below in
Antenna type : Directional (Unidirectional or Bidirectional)
Frequency : 30 – 1800 MHz
Polarization : horizontal / vertical
Gain : 6 – 8 dBi
Impedance : 50 Ohms
VSWR : ≤ 1.5:1
Elevation 3 dB beamwidth : ≤45°
Azimuth 3 dB Beamwidth : ≤50°
First sidelobe level : ≤-20 dB
Front to back ratio : ≤-25 dB
Boom Length : 1050mm
Mast Size : 30-50mm
Weight : 2,3kg
Element Length : 1110mm max
Power Rating : 50 W
Lightning protection : DC grounded
below show a few types of practical LPDA Antenna.
Figure 1.1 - LPDA
Figure 1.2 - Horizontal
plane radiation patterns
LOG PERIODIC DIPOLE ARRAY